Not applicable.
Prostate cancer accounts for about 1 in 10 cancer cases in the United States, and it is the most often diagnosed cancer in males [Henderson et al. (1991) Science 254,1131-1138]. While in many affected patients, the tumors are slow-growing and nonmetastatic, in others the malignant prostate tumors are aggressive and metastasize. When prostate cancer metastasizes, the prognosis for the patient is poor, especially without treatment.
To date, the most frequently used test for prostate cancer is the serum level of prostate specific antigen (PSA) and the radionuclide bone scan for detecting prostate cancer metastases before definitive therapy is initiated. However, the elevated level of PSA in serum is not predictive of the pathologic stage of the prostate cancer or the presence of metastatic disease. PSA, a serine protease, is not exclusively expressed in the epithelial cells of metastatic prostate cancer, but it is also expressed in normal epithelial cells, primary tumors and benign prostate hyperplasia.
The altered expression of cell-adhesion molecules has been correlated with metastasis of many cancers. Low or no expression of E-cadherin, a cell-adhesion molecule, has been found in high-grade prostate carcinoma, and this indicates a poor prognosis for those prostate cancer patients. However, the absence of an antigen is not very useful as a diagnostic marker for cancer metastasis.
MUC18 is a glycoprotein of about 113 kDa which serves as a cell adhesion molecule on the surface of melanoma cells, and it has been correlated with the ability of melanomas to metastasize [See, e.g., Lehmann et al. (1989) Proc. Nat. Acad. Sci. USA 86, 9891-9895; Luca et al. (1993) Melanoma Res. 3, 3541; Johnson et al. (1996) Curr. Top. Microbiol. Immunol. 213, 95-105; Xie et al. (1997) Cancer Res. 57, 2295-2303; Tang and Honn (1994-1995) Invasion Metas. 14, 109-122; Rummel et al. (1996) Cancer Res. 56, 2218-2223]. MUC18 is also known as MCAM and CD146. MUC18 carries a carbohydrate modification known as HNK-1 or CD57 [Shih et al. (1994) Cancer Res. 54, 2514-2520]. Besides being associated with melanoma cells"" ability to metastasize, MUC18 is also associated with normal vascular tissue, and on the smooth muscle of venules, and it expresses sporadically on capillary epithelium [Johnson, J. (1994-1995) Invasion Metas. 14, 123-130].
There is a longfelt need in the art for an improved diagnostic test for metastatic prostate cancer so that appropriate therapy can be initiated as soon as possible and so that the number of false positive results can be minimized.
The present invention provides an improved diagnostic test for prostate cancer which has a relatively high potential for metastasis or which has metastasized. This allows the physician to choose appropriate surgical, chemotherapeutic or radiation treatment regimens. This improved assay is based on the correlation of high levels of expression of the MUC18 coding sequence as measured by MUC18 mRNA or MUC18 protein. This expression can be detected at the transcriptional level, where mRNA levels are monitored, or detection of the MUC18 gene product at the translation level can be determnined, for example, through the use of an immunoassay for the MUC18 protein. The source of the material for these tests is prostate biopsy tumor tissue (e.g., from a needle biopsy) from a patient needing a determination of the metastatic potential of a prostate tumor or from cells from a prostate tumor.
Relative levels of transcriptional expression (mRNA) of the MUC18 coding sequence can be determined by Northern hybridization analysis or by quantitative reverse transcription polymerase chain reaction (RT-PCR) in normal and neoplastic prostate tissue samples and in biopsy material.
Translational expression of MUC18 can be determined by any of a number of adaptations of an immunoassay using antibody specific for the MUC18 cell surface antigen. The relative level of MUC18 can be determined by standard immunoassays using a MUC18-specific antibody preparation and a detection system suitable for the assay. Immunoassays can include, but are not limited to, immunofluorescence assays, radioimmunoassay, enzyme-linked immunosorbent assays, and Western (immuno) blot assays. In the context of the present invention, relative amounts of the MUC18 protein are determined in tissue samples (e.g., biopsy material).
It is a further object of the present invention to provide an antibody which inhibits prostate cancer metastasis. In particular, antibody specific to MUC18 prevents metastasis of prostate cancer cells.
Additional objects include vectors directing the expression of an immunogenic fragment of human MC18and the corresponding recombinantly expressed protein. As specifically exemplified, an immunogenic fragment of human MUC18 is encoded by the PvuII to XhoI fragment within the sequence given in Tables 1A-1B.